1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and an image forming method, an image forming apparatus and process cartridge for image forming apparatus using the electrophotographic photoreceptor.
2. Discussion of the Background
Recently, information-processing systems using an electrophotographic method are making a remarkable progress. In particular, laser printers and digital copiers which record data with light by changing the data into digital signals make remarkable improvements in their printing qualities and reliabilities. Further, technologies used in these printers and copiers are applied to laser printers and digital copiers capable of printing full-color images with high-speed printing technologies. Because of these reasons, photoreceptors are required both to produce high-quality images and to have high durability.
Photoreceptors using organic photosensitive materials are widely used for these laser printers and digital copiers due to their cost, productivity and non-polluting properties. The organic photoreceptors are generally classified to a single-layered type and a functionally-separated type. The first practical organic photoreceptor, i.e., PVK-TNF charge transfer complex photoreceptor was the former single-layered type. In 1968, Mr. Hayashi and Mr. Regensburger independently invented PVK/a-Se multi-layered photoreceptor. In 1977, Mr. Melz, and in 1978, Mr. Schlosser disclosed a multi-layered photoreceptor whose photosensitive layers are all formed from organic materials, i.e. , an organic-pigment dispersed layer and an organic low-molecular-weight material dispersed polymer layer. These are called as functionally-separated photoreceptors because of having a charge generation layer (CGL) generating a charge by absorbing light and a charge transport layer (CTL) transporting the charge and neutralizing the charge on a surface of the photoreceptor. The multi-layered photoreceptor has much more improved sensitivity and durability than the single-layered photoreceptor. In addition, since materials can be separately selected for a charge generation material (CGM) and a charge transport material (CTM), a choice range of the materials is largely expanded. Because of these reasons, the multi-layered photoreceptor is now prevailing in the market.
A mechanism to form an electrostatic latent image in the multi-layered photoreceptor is as follows:
the photoreceptor is charged and irradiated with light; the light passes through the CTL and is absorbed by the CGM in the CGL to generate a charge; the charge is injected into the CTL at an interface of the CGL and the CTL; and the charge moves in the CTL by an electric field and neutralizes the charge on the surface of the photoreceptor to form an electrostatic latent image.
However, the photosensitive layers of the organic photoreceptor are easily abraded due to repeated use, and therefore potential and photosensitivity of the photoreceptor tend to deteriorate, resulting in background fouling due to a scratch on the surface thereof and deterioration of density and quality of the resultant images. Therefore, abrasion resistance of the organic photoreceptor has been an important subject. Further, recently, in accordance with speeding up of the printing speed and downsizing of an image forming apparatus, the photoreceptor has to have a smaller diameter, and durability thereof becomes a more important subject.
As a method of improving the abrasion resistance of the photoreceptor, methods of imparting lubricity to the photosensitive layer, hardening the photosensitive layer, including a filler therein and using a high-molecular-weight CTM instead of a low-molecular-weight CTM are widely known. However, another problem occurs when these methods are used to prevent the abrasion of the photoreceptor. Namely, an oxidized gas such as ozone and NOx arising due to use conditions or environment, adheres to the surface of the photosensitive layer and decreases the surface resistance thereof, resulting in a problem such as blurring of the resultant images. So far, such a problem has been avoided to some extent because the material causing the blurred images are gradually scraped off in accordance with the abrasion of the photosensitive layer. However, in order to comply with the above-mentioned recent demand for higher sensitivity and durability of the photoreceptor, a new technique has to be imparted thereto. In order to decrease an influence of the material causing the blurred images, there is a method of equipping the photoreceptor with a heater, which is a large drawback for downsizing the apparatus and decreasing the electric power consumption. In addition, a method of including an additive such as an antioxidant in the photosensitive layer is effective, but since a simple additive does not have photoconductivity, including much amount thereof in the photosensitive layer causes problems such as deterioration of the sensitivity and increase of residual potential of the resultant photoreceptor.
As mentioned above, the electrophotographic photoreceptor having less abrasion by being imparted with abrasion resistance or a process design around thereof inevitably produces blurred and low-resolution images, and it is difficult to have both of high durability and high quality of the resultant images. This is because high surface resistance of the photosensitive layer is preferable to prevent the blurred images and low surface resistance thereof is preferable to prevent the increase of residual potential.
Japanese Laid-Open Patent Publication No. 2000-231204 or 2002-313111 discloses a method of including at least a compound having a dialkylamino group in a photosensitive layer to solve the above-mentioned problem such as blurring of the resultant images due to a blur generating material such as an oxidizing gas. The reason why the compound is effective for maintaining quality of the resultant images after repeated use is not clarified at this time. However, it is supposed that the dialkylamino group having a strong basic neutralizes the oxidizing gas which is considered to cause the blurred images. However, the compound has an effect on image quality after the repeated use, but the resultant photoreceptor does not have high sensitivity and cannot comply with high speed printing because of having a low charge transportability. Therefore, an addition amount thereof has a limit, and a method of combining the compound with a CTM to increase sensitivity and repeated use stability of the resultant photoreceptor is disclosed therein.
On the other hand, it is described that a stilbene compound having a dialkylamino group disclosed in Japanese Laid-Open Patent Publication No. 60-196768 and Japanese Patent No. 2884353 has an effect on the blurred images due to the oxidizing gas on page 37 of Konica Technical Report Vol. 13 written by Itami, etc. and published in 2000. However, since the compound has a substituted dialkylamino group having a strong mesomeric effect (+M effect) at a resonance portion in its triarylamine structure, which is a charge transport site, total ionization potential is extremely small. Therefore, the compound has a critical defect of being quite difficult to practically use because charge retainability of a photosensitive layer in which the compound is used alone as a CTM largely deteriorates from the beginning or after repeated use. In addition, even when the above-mentioned stilbene compound is used together with other CTMs as it is in the present invention, the compound has a considerably smaller ionization potential than the other CTMs and becomes a trap site against a charge transport, and therefore, the resultant photoreceptor has quite a low sensitivity and a large residual potential.
Because of these reasons, a need exists for an electrophotographic photoreceptor having high durability against repeated use for a long time, preventing deterioration of image density and blurred images and stably producing quality images.